Delve vs. Stubborn Denial

As you may know, Delve and Convoke’s basic workings were changed with the rules updates for Magic 2014. Previously, they were simply ways to reduce costs before you paid them; nowadays, the Delving/Convoking is a payment! The only real difference, functionality-wise, is that you can no longer “over-delve” or “over-convoke”- since it’s a payment, at some point you run out of colored mana cost to pay. This is, for example, why Souflayer can’t just eat 10 or 11 creatures to gain a whole mess of keywords, but can only eat 4 without help. However, even though Delve is a payment now, it’s ONLY a payment for properly casting the spell. If your opponent throws a Stubborn Denial or Temur Charm at your Delve spell, you can’t Delve to pay off the counterspell- it isn’t part of the Delve spell’s cost, so the Delve ability won’t apply. What’s worse, by the time your opponent is casting their counterspell, your Delve spell is fully cast. You can’t go back and decide to exile 4 cards instead of just 1 so you can pay for the Charm- it’s too late!